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    SubjectRe: [RFC] respect the referenced bit of KVM guest pages?
    On Wed, Aug 05, 2009 at 10:15:16AM -0400, Rik van Riel wrote:
    > Not having a hardware accessed bit would explain why
    > the VM is not reactivating the pages that were accessed
    > while on the inactive list.

    Problem is, even with young bit functional the VM isn't reactivating
    those pages anyway because of that broken check... That check should
    be nuked entirely in my view as it fundamentally thinks it can
    outsmart the VM intelligence by checking a bit in the vma... quite
    absurd in my view.

    > Can we find out which pages are EPT pages?
    > If so, we could unmap them when they get moved from the
    > active to the inactive list, and soft fault them back in
    > on access, emulating the referenced bit for EPT pages and
    > making page replacement on them work like it should.
    > Your approximation of pretending the page is accessed the
    > first time and pretending it's not the second time sounds
    > like it will just lead to less efficient FIFO replacement,
    > not to anything even vaguely approximating LRU.

    I think it'll still better than current situation, as young bit is
    always set for ptes. Otherwise EPT pages are too penalized, we need
    them to stay one round more in active list like everything else. They
    are too penalizied anyways because at second pass they'll be forced
    out of active list and unmapped.

    This is what alpha and all other archs without young bit set in
    hardware have to do. They set young bit in software and clear it in
    software and then set it again in software if there's a page fault
    (hopefully a minor fault). Returning "not young" first time sounds
    worse to me.

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      Last update: 2009-08-05 18:35    [W:0.029 / U:6.116 seconds]
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